The preparation of styrenic block copolymers (“SBCs”) is well known in the art. Generally, SBCs can comprise internal polymer blocks and terminal or end polymer blocks comprising chemically different monomer types thereby providing particular desirable properties. As an example, in a more common form, SBCs may have internal blocks of conjugated diene, or the hydrogenated counterparts thereof, and terminal blocks having aromatic alkenyl arenes. The interaction of the differing properties of the polymer blocks allow for different polymer characteristics to be obtained. For example, the elastomer properties of internal conjugated diene blocks, or the hydrogenated counterparts thereof, along with the “harder” aromatic alkenyl arenes terminal blocks together form polymers which are useful for an enormous variety of applications. Such SBCs can be prepared through sequential polymerization and/or through coupling reactions.
It is known also that SBCs can be functionalized in order to further modify their characteristics. An example of this is the addition of sulfonic acid or sulfonate ester functional groups to the polymer backbone. One of the first such sulfonated block copolymers is disclosed, for example, in U.S. Pat. No. 3,577,357 to Winkler. The sulfonated block copolymer was characterized as having the general configuration A-B-(B-A)1-5, wherein each A is a non-elastomeric sulfonated monovinyl arene polymer block and each B is a substantially saturated elastomeric alpha-olefin polymer block, the block copolymer being sulfonated to an extent sufficient to provide at least 1% by weight of sulfur in the total polymer and up to one sulfonate group per monovinyl arene unit. The sulfonated polymers can be used as such or in the form of their acid, alkali metal salt, ammonium salt or amine salt. According to Winkler, a polystyrene-hydrogenated polyisoprene-polystyrene triblock copolymer was treated with a sulfonating agent comprising sulfur trioxide/triethyl phosphate in 1,2-dichloroethane. The sulfonated block copolymers are described as having water absorption characteristics useful in water purification membranes and the like. Further sulfonated SBCs are disclosed or mentioned, e.g., in U.S. Pat. No. 3,642,953, U.S. Pat. No. 3,870,841, U.S. Pat. No. 4,492,785, U.S. Pat. No. 4,505,827, U.S. Pat. No. 5,239,010, U.S. Pat. No. 5,468,574, U.S. Pat. No. 5,516,831, and U.S. Pat. No. 7,169,850.
More recently, U.S. Pat. No. 7,737,224 to Willis et al., has described the preparation of sulfonated polymers and inter alia illustrated sulfonated block copolymers which are solid in water comprising at least two polymer end blocks and at least one polymer interior block wherein each end block is a polymer block resistant to sulfonation and at least one interior block is a polymer block susceptible to sulfonation, and wherein at least one interior block is sulfonated to the extent of 10 to 100 mol percent of the sulfonation susceptible monomer unit in the block. The sulfonated block copolymers are described as being able to transport high amounts of water vapor while at the same time having good dimensional stability and strength in the presence of water.
US 2010/0048817 to Dado et al. is illustrative of further developments in the manufacture of sulfonated block copolymers and describes a process for preparing sulfonated block copolymers illustrating, e.g., the sulfonation of a precursor block polymer having at least one end block A and at least one interior block B wherein each A block is a polymer block resistant to sulfonation and each B block is a polymer block susceptible to sulfonation wherein said A and B blocks are substantially free of olefinic unsaturation. The precursor block polymer was reacted with an acyl sulfate in a reaction mixture further comprising at least one non-halogenated aliphatic solvent. According to Dado et al., the process results in a reaction product which comprised micelles of sulfonated polymer and/or other polymer aggregates of definable size and distribution. More recently, U.S. Pat. No. 8,012,539 to Handlin et al., disclosed an improved process for preparing sulfonated block copolymers and esters thereof, as well as membranes comprising them.
Carbon black, carbon nanotubes, and fullerenes have been proposed among others as fillers or reinforcing agents for sulfonated block copolymers (e.g., U.S. Pat. No. 8,012,539, U.S. Pat. No. 7,737,224, U.S. Pat. No. 5,516,831, and U.S. Pat. No. 5,239,010). Additionally, it has been proposed to disperse metal impregnated carbon particles in sulfonated block copolymers to obtain electrode assemblies (e.g., U.S. Pat. No. 7,737,224, and U.S. Pat. No. 8,012,539).
However, the manufacture of composites comprising carbon and a polymer can be challenging as the polymer has to wet, and adhere to, the carbon particles. For example, perfluoroalkylenesulfonic acid polymer compounds, such as NAFION® available from DuPont, have been used widely as ion-conductive polymer binders of electrode catalyst layers employed, e.g., in fuel cells (e.g., U.S. Pat. No. 4,876,115). NAFION® is commercially available as a slurry in water. However, such slurries do not wet carbon well, and mixing carbon with the binder polymer is problematic. In addition, the adhesion of the respective sulfonic acid polymer to carbon is poor.
It has now been found that the sulfonated block copolymers specified at the outset are surprisingly well suited to disperse particulate carbon and form compositions having uniform consistency. When employing a suitable dispersant, the compositions form liquid dispersions having unexpectedly low viscosity which can easily be cast into films and membranes, and which can easily be coated on a broad variety of substrates. Moreover, the films, membranes and coatings comprising the compositions of the present disclosure are dimensionally stable in the presence of water, and surprisingly effective in transporting water vapor, heat and electricity.